Method and apparatus for transmitting sounding reference signal

ABSTRACT

A method is provided for receiving a sounding reference signal (SRS) in a wireless communication system. A base station transmits downlink control information to a user equipment at a first serving cell of the plurality of serving cells. The downlink control information includes a carrier indicator and a downlink assignment. When the downlink control information includes an aperiodic SRS request that indicates a triggering of an SRS transmission, the base station receives an SRS from the user equipment at the second serving cell. If the first serving cell is a frequency division duplex (FDD) and the second serving cell is a time division duplex (TDD) cell, the downlink control information includes the aperiodic SRS request. If the first serving cell is the TDD cell and the second serving cell is the FDD cell, the downlink control information does not include the aperiodic SRS request.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a Continuation of U.S. patent application Ser. No.14/604,284 filed on Jan. 23, 2015 (now U.S. Pat. No. 9,450,724 issued onSept. 20, 2016), which claims the benefit under 35 U.S.C. §119(e) toU.S. Provisional Application Nos. 62/001,591 filed on May 21, 2014 and61/930,958 filed on Jan. 24, 2014, and under 35 U.S.C. §119(a) to KoreanPatent Application No. 10-2014-0147349 filed on Oct. 28, 2014, all ofwhich are hereby expressly incorporated by reference into the presentapplication.

BACKGROUND OF THE INVENTION

The present invention relates to a wireless communication, and moreparticularly, to a sounding reference signal transmission performed by awireless device to which a plurality of serving cells are configured.

In a wireless communication system, a duplex scheme includes a timedivision duplex (TDD) and a frequency division duplex (FDD). The TDDprovides an uplink communication and a downlink communication in thesame frequency band. The FDD provides the uplink communication and thedownlink communication in different frequency bands.

A carrier aggregation is a technique capable of providing a plurality ofcomponent carriers to a user equipment. Each component carrier may bedefined as one cell. When the plurality of component carriers areconfigured to the user equipment, it can be regarded that the userequipment receives services from a plurality of serving cells.

Each serving cell may be configured with the FDD or the TDD. In acarrier aggregation environment, the user equipment may be configuredwith a plurality of duplex schemes. For example, if two cells areconfigured to the user equipment, the cells may be configured as a TDDcell-TDD cell or a TDD cell-FDD cell. Accordingly, a throughput of theuser equipment can be increased in various network environments.

The conventional uplink signal transmission does not consider anenvironment in which a plurality of serving cells using different duplexschemes are configured.

SUMMARY OF THE INVENTION

The present invention provides a method of transmitting a soundingreference signal by a wireless device to which a plurality of servingcells using different duplex schemes are configured.

In an aspect, a method for transmitting a sounding reference signal(SRS) in a wireless communication system is provided. The method isperformed by a user equipment that is configured with a plurality ofserving cells including a time division duplex (TDD) cell and afrequency division duplex (FDD) cell. The method includes receivingdownlink control information from a first serving cell of the pluralityof serving cells, the downlink control information including a carrierindicator and a downlink assignment, the carrier indicator indicating asecond serving cell of the plurality of serving cells, the downlinkassignment indicating a downlink resource assignment at the secondserving cell, and transmitting a SRS through the second serving cell ifthe downlink control information includes the aperiodic SRS request andthe aperiodic SRS request indicates a triggering of a SRS transmission.

If the first serving cell is the FDD cell and the second serving cell isthe TDD cell, the downlink control information may include the aperiodicSRS request. If the first serving cell is the TDD cell and the secondserving cell is the FDD cell, the downlink control information may notinclude the aperiodic SRS request.

The first serving cell may be a primary cell and the second serving cellmay be a secondary cell.

In another aspect, a device configured to transmit a sounding referencesignal (SRS) in a wireless communication system and configured with aplurality of serving cells including a time division duplex (TDD) celland a frequency division duplex (FDD) cell is provided. The deviceincludes a radio frequency (RF) unit configured to receive and transmitradio signals; and a processor operatively coupled with the RF unit andconfigured to instruct the RF unit to receive downlink controlinformation from a first serving cell of the plurality of serving cells,the downlink control information including a carrier indicator and adownlink assignment, the carrier indicator indicating a second servingcell of the plurality of serving cells, the downlink assignmentindicating a downlink resource assignment at the second serving cell,and instruct the RF unit to transmit a SRS through the second servingcell if the downlink control information includes the aperiodic SRSrequest and the aperiodic SRS request indicates a triggering of a SRStransmission.

When a plurality of serving cells having different duplex schemes areconfigured, an uplink channel estimation can be improved by using asounding reference signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a structure of a downlink (DL) radio frame in 3rdgeneration partnership project (3GPP) long term evolution(LTE)/LTE-advanced (LTE-A).

FIG. 2 shows an example of an uplink (UL) subframe.

FIG. 3 shows an example in which a plurality of serving cells areconfigured.

FIG. 4 is a flowchart showing a sounding reference signal (SRS)transmission according to an embodiment of the present invention.

FIG. 5 is a block diagram showing a wireless communication systemaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A wireless device may be fixed or mobile, and may be referred to asanother terminology, such as a user equipment (UE), a mobile station(MS), a user terminal (UT), a subscriber station (SS), a personaldigital assistant (PDA), a wireless modem, a handheld device, etc. Thewireless device may also be a device supporting only data communicationsuch as a machine-type communication (MTC) device.

A base station (BS) is generally a fixed station that communicates withthe wireless device and may be referred to as another terminology, suchas an evolved node-B (eNB), a base transceiver system (BTS), an accesspoint, etc.

Hereinafter, the present invention is applied based on a 3rd generationpartnership project (3GPP) long term evolution (LTE) or a 3GPPLTE-Advanced (LTE-A). This is for exemplary purposes only, and thus thepresent invention is applicable to various communication systems.

A wireless device may be served by a plurality of serving cells. Eachserving cell may be defined with a downlink (DL) component carrier (CC)or a pair of a DL CC and an uplink (UL) CC.

Each serving cell may be configured as a time division duplex (TDD) or afrequency division duplex (FDD). A TDD cell is a cell configured to theTDD. A FDD cell is a cell configured to the FDD.

A serving cell may be classified into a primary cell and a secondarycell. The primary cell operates at a primary frequency, and is a celldesignated as the primary cell when an initial network entry process isperformed or when a network re-entry process starts or in a handoverprocess. The primary cell is also called a reference cell. The secondarycell operates at a secondary frequency. The secondary cell may beconfigured after an RRC connection is established, and may be used toprovide an additional radio resource. At least one primary cell isconfigured always. The secondary cell may be added/modified/released byusing higher-layer signaling (e.g., a radio resource control (RRC)message).

A cell index (CI) of the primary cell may be fixed. For example, alowest CI may be designated as the CI of the primary cell. It is assumedhereinafter that the CI of the primary cell is 0 and a CI of thesecondary cell is allocated sequentially starting from 1.

FIG. 1 shows a downlink radio frame structure in 3GPP LTE/LTE-A.

A radio frame includes 10 subframes indexed with 0 to 9. One subframeincludes 2 consecutive slots. A time required for transmitting onesubframe is defined as a transmission time interval (TTI). For example,one subframe may have a length of 1 millisecond (ms), and one slot mayhave a length of 0.5 ms.

One slot may include a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain. Since the 3GPP LTE usesorthogonal frequency division multiple access (OFDMA) in a downlink(DL), the OFDM symbol is only for expressing one symbol period in thetime domain, and there is no limitation in a multiple access scheme orterminologies. For example, the OFDM symbol may also be referred to asanother terminology such as a single carrier frequency division multipleaccess (SC-FDMA) symbol, a symbol period, etc.

Although it is described that one slot includes 7 OFDM symbols forexample, the number of OFDM symbols included in one slot may varydepending on a length of a cyclic prefix (CP). According to 3GPP TS36.211 V8.7.0, in case of a normal CP, one slot includes 7 OFDM symbols,and in case of an extended CP, one slot includes 6 OFDM symbols.

A resource block (RB) is a resource allocation unit, and includes aplurality of subcarriers in one slot. For example, if one slot includes7 OFDM symbols in a time domain and the RB includes 12 subcarriers in afrequency domain, one RB can include 7×12 resource elements (REs).

A special (S) subframe is a unique kind of subframe used for the TDD. Asubframe having an index #1 and an index #6 is called a S subframe. TheS subframe includes a downlink pilot time slot (DwPTS), a guard period(GP), and an uplink pilot time slot (UpPTS). The DwPTS is used in the UEfor initial cell search, synchronization, or channel estimation. TheUpPTS is used in the BS for channel estimation and uplink transmissionsynchronization of the UE. The GP is a period for removing interferencewhich occurs in an uplink due to a multi-path delay of a downlink signalbetween the uplink and a downlink.

In TDD, a downlink (DL) subframe and an uplink (UL) subframe co-exist inone radio frame. Table 1 shows an example of a configuration of theradio frame.

TABLE 1 UL-DL Switch-point Subframe index configuration periodicity 0 12 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5 ms D S U U D D S U U D 25 ms D S U D D D S U D D 3 10 ms D S U U U D D D D D 4 10 ms D S U U D DD D D D 5 10 ms D S U D D D D D D D 6 5 ms D S U U U D S U U D ‘D’denotes a DL subframe, ‘U’ denotes a UL subframe, and ‘S’ denotes aspecial subframe. When the UL-DL configuration is received from the BS,the UE can know whether a specific subframe is the DL subframe or the ULsubframe according to the configuration of the radio frame.

A DL subframe may be divided into a control region and a data region inthe time domain. The control region includes up to first four OFDMsymbols of a 1st slot in the subframe. However, the number of OFDMsymbols included in the control region may vary. A physical downlinkcontrol channel (PDCCH) and other control channels are allocated to thecontrol region, and a physical downlink shared channel (PDSCH) isallocated to the data region.

Hereinafter, DL control channels are disclosed.

Examples of physical control channels in 3GPP LTE/LTE-A include aphysical downlink control channel (PDCCH), a physical control formatindicator channel (PCFICH) and a physical hybrid-ARQ indicator channel(PHICH).

The PCFICH transmitted in a 1st OFDM symbol of the subframe carries acontrol format indicator (CFI) regarding the number of OFDM symbols(i.e., a size of the control region) used for transmission of controlchannels in the subframe. The UE first receives the CFI on the PCFICH,and thereafter monitors the PDCCH.

Unlike the PDCCH, the PCFICH is transmitted by using a fixed PCFICHresource of the subframe, without having to perform blind decoding.

The PHICH carries a positive-acknowledgement(ACK)/negative-acknowledgement (NACK) signal for an uplink hybridautomatic repeat request (HARQ). The ACK/NACK signal for uplink (UL)data on a PUSCH transmitted by the UE is transmitted on the PHICH.

A physical broadcast channel (PBCH) is transmitted in first four OFDMsymbols in a 2nd slot of a 1st subframe of a radio frame. The PBCHcarries system information necessary for communication between the UEand the BS. The system information transmitted through the PBCH isreferred to as a master information block (MIB). In comparison thereto,system information transmitted on the PDCCH indicated by the PDCCH isreferred to as a system information block (SIB).

Control information transmitted through the PDCCH is referred to asdownlink control information (DCI). The DCI may include resourceallocation of the PDSCH (this is referred to as a DL grant), resourceallocation of a PUSCH (this is referred to as a UL grant), a set oftransmit power control commands for individual UEs in any UE group,and/or activation of a voice over Internet protocol (VoIP).

In 3GPP LTE/LTE-A, blind decoding is used to detect a PDCCH. Blinddecoding is a scheme in which a desired identifier is demasked to areceived PDCCH (this is called candidate PDCCH) and a CRC error ischecked to thereby verify whether the PDCCH is its own control channel.

A base station determines a PDCCH format depending on a DCI that is tobe sent to the wireless device, adds a CRC (cyclic redundancy check) tothe DCI, and masks to the CRC a unique identifier (this is called RNTI(radio network temporary identifier) depending on the owner or purposeof the PDCCH.

A search space is used to lessen burden due to blind decoding. Thesearch space may be a CCE's monitoring set for the PDCCH. The wirelessdevice monitors the PDCCH in the corresponding search space.

Search spaces are divided into a common search space and a UE-specificsearch space. The common search space is a space for searching a PDCCHhaving common control information and consists of 16 CCEs indexed 0 to15 while supporting a PDCCH having a CCE aggregation level of {4, 8}.However, a PDCCH (DCI format 0, 1A) conveying UE-specific informationmay also be monitored in the common search space. The UE-specific searchspace supports a PDCCH having a CCE aggregation level of {1, 2, 4, 8}.

FIG. 2 shows an example of a UL subframe. The number of OFDM symbolsincluded in the subframe and an arrangement of each UL channel are forexemplary purposes only.

UL channels include a Physical Uplink Shared Channel (PUSCH), a PhysicalUplink Control Channel (PUCCH), a sounding reference signal (SRS), and aphysical random access channel (PRACH).

The PUSCH may be allocated by a UL grant on a PDCCH, and is used tocarry user traffic. When the PUSCH is transmitted, a demodulationreference signal (DM RS) for PUSCH demodulation is transmitted in a 4thOFDM symbol of each slot. The PUCCH is used to carry a control signalsuch as CQI and ACK/NACK for HARQ.

The SRS is a signal used by a BS to estimate a state of the UL channel.

The SRS transmission can be classified into a periodic SRS transmissionand an aperiodic SRS transmission. The periodic SRS transmission occursin a subframe triggered by a periodic SRS configuration. The periodicSRS configuration includes an SRS periodicity and an SRS subframeoffset. If the periodic SRS configuration is given, a UE canperiodically transmit an SRS in a subframe satisfying the periodic SRSconfiguration.

In the aperiodic SRS transmission, the SRS is transmitted upon detectionof an SRS request of a BS. For the aperiodic SRS transmission, the SRSconfiguration is given. The SRS configuration also includes an SRSperiodicity T_(SRS) and an SRS subframe offset T_(offset).

The SRS request for triggering of the aperiodic SRS transmission may beincluded in a DL grant or a UL grant on a PDCCH. For example, if the SRSrequest has 1 bit, ‘0’ may indicate a negative SRS request, and ‘1’ mayindicate a positive SRS request. If the SRS request has 2 bits, ‘00’ mayindicate a negative SRS request, and the others may indicate a positiveSRS request. In this case, one of a plurality of SRS configurations forSRS transmission can be selected.

Assume that a positive SRS request is detected in a subframe n of aserving cell. Upon detection of the positive SRS request, an SRS istransmitted in a first subframe satisfying a condition of n+k where k≧4as well as T_(SRS)>2 in TDD and (10*nf+k_(SR)S−T_(off)t) mod T_(SRS)=0in FDD. In FDD, a subframe index k_(SRS) is {0,1, . . ,9} in a frame nf.In TDD, k_(SRS) is defined by a predetermined table. In TDD ofT_(SRS)=2, the SRS is transmitted in a first subframe satisfying acondition of (k_(SRS)−T_(offset))mod5=0.

Hereinafter, a subframe in which the SRS is transmitted is called an SRSsubframe or a triggered subframe. In periodic SRS transmission andaperiodic SRS transmission, the SRS can be determined in an SRS subframedetermined UE-specifically.

An OFDM symbol in which the SRS is transmitted may have a fixed positionin the SRS subframe. For example, the SRS may be transmitted in a lastOFDM symbol of the SRS subframe. The OFDM symbol in which the SRS istransmitted is called a sounding reference symbol.

Now, an UL signal transmission proposed in a carrier aggregation (CA)environment in which a plurality of serving cells are configured isdescribed.

FIG. 3 shows an example in which a plurality of serving cells areconfigured.

It is assumed that an FDD cell and a TDD cell are configured to awireless device, and the FDD cell is a primary cell and the TDD cell isa secondary cell. It is also assumed that the TDD cell is activated bysignaling of the FDD cell. The number of configured cells is forexemplary purposes only.

A method of scheduling a plurality of serving cells includes noncross-carrier scheduling and cross-carrier scheduling. In the noncross-carrier scheduling, a scheduled cell is identical to a schedulingcell. In the cross-carrier scheduling, the scheduled cell is differentfrom the scheduling cell. The scheduling cell is a cell in which ascheduling signal is received, that is, a cell in which a PDCCH ismonitored, and is also called a monitoring cell. The scheduled cell is acell in which a PDSCH scheduled by a PDCCH is received (or in which aPUSCH is transmitted).

FIG. 3 shows an example of cross-carrier scheduling. An FDD cell is ascheduling cell, and a TDD cell is a scheduled cell. It is assumed thata subframe n is a special (S) subframe or a downlink (DL) subframe inthe TDD cell. A wireless device receives downlink control information(DCI) on a PDCCH 310 in the subframe n of the FDD cell, and receives DLdata on a PDSCH 320 of the TDD cell indicated by the DCI.

Whether to perform the cross-carrier scheduling may be given to thewireless device by signaling of a primary cell. If the cross-carrierscheduling is possible, the DCI may include a carrier indicator field(CIF) in addition to a DL resource allocation for the PDSCH. The CIF mayindicate an index of a scheduled cell.

Meanwhile, as described above, the DCI may include a SRS request fortriggering an aperiodic SRS transmission. This is to confirm a ULchannel state of the wireless device by a serving cell even at a time inwhich a periodic SRS transmission is not achieved.

It is assumed that the SRS request in the DCI on the PDCCH 310 triggersa SRS transmission. It is also assumed that a subframe n+5 is a SRSsubframe which satisfies an aperiodic SRS transmission configuration.The wireless device transmits a SRS 330 in the subframe n+5 of a TDDcell.

FIG. 4 is a flowchart showing a SRS transmission according to anembodiment of the present invention. It is assumed that a TDD cell andan FDD cell are configured to a wireless device.

In step S410, the wireless device receives DCI from a first serving cell(i.e., a scheduling cell). The DCI may be received on a PDCCH of thefirst serving cell. The DCI may include a CFI indicating a secondserving cell (i.e., a scheduled cell) and a DL resource allocation.

In step S420, the wireless device determines whether the SRS request isincluded in the DCI. The wireless device may determine whether the SRSrequest is included in the DCI according to a duplex scheme of the firstserving cell and the second serving cell.

Irrespective of the duplex scheme of the first serving cell, thewireless device may determine that the SRS request is included in theDCI if the second serving cell is a TDD cell. If the second serving cellis not the TDD cell, the wireless device may determine that the SRSrequest is not included in the DCI. More specifically, if the firstserving cell is an FDD cell and the second serving cell is the TDD cell,the wireless device may determine that the SRS request is included inthe DCI. If the first serving cell is the TDD cell and the secondserving cell is the FDD cell, the wireless device may determine that theSRS request is not included in the DCI.

In step S430, if it is determined that the SRS request is included inthe DCI, it is determined whether the SRS request triggers a SRStransmission.

The SRS request may be fixed to a size of one bit. For example, if theSRS request has a value of ‘1’, the SRS transmission is requested, andif the SRS request has a value of ‘0’, the SRS transmission is notrequested.

If the SRS transmission is triggered, in step S440, the wireless devicetransmits a SRS in a SRS subframe. Before that, the wireless device mayreceive an aperiodic SRS configuration for determining the SRS subframefrom a BS.

In step S450, the wireless device receives DL data on a PDSCH on theDCI. If the DCI is received in a subframe n, the DL data is alsoreceived in the subframe n. On the other hand, the SRS is transmitted ina subframe n+k (k>1). This step may be performed before the step S420,or may be performed simultaneously with the step S420.

When whether the SRS request is included in the DCI having a DLallocation (this is called DL scheduling DCI) is determined according tothe duplex scheme of the serving cell, the following advantages areprovided.

First, in a TDD cell, a DL/UL transmission is divided in a time durationand thus a transmission opportunity is relatively small in comparisonwith the FDD cell. Therefore, it is not easy to distribute a periodicSRS to all wireless devices in a cell while avoiding a collision.Accordingly, a method in which the SRS is transmitted only whennecessary such as in an aperiodic SRS may be effective in the TDD cell.

Second, in the TDD cell, the same frequency carrier is used in a DLtransmission and a UL transmission, and thus a similarity of a DLchannel and a UL channel exists. Accordingly, a SRS may be used tomeasure DL channel quality for DL scheduling. In a case where DLscheduling is relatively more frequent than UL scheduling in a specificdevice, it may be effective that a request of a SRS transmission isallowed to estimate DL channel quality for the DL scheduling.

Third, in the TDD cell, a UL transmission opportunity is relativelysmaller than the FDD cell, and thus an opportunity for transmitting DCIhaving a UL grant is decreased. Therefore, even if the UL scheduling isnot performed, it is necessary to trigger an aperiodic SRS transmissionthrough the DCI.

A configuration regarding whether to determine whether a SRS request isincluded in DL DCI for scheduling the TDD cell may be given by RRCsignaling. When the configuration is activated, the decision of the stepS420 may be performed.

In order to increase an opportunity capable of triggering the aperiodicSRS, if a DL scheduled cell is the TDD cell or a UL scheduling cell isthe TDD cell, the DCI may be allowed to include the aperiodic SRS. Thisis because if a cell for transmitting the SRS is the TDD cell or if acell for transmitting DCI having a UL grant is the TDD cell, anopportunity capable of transmitting the UL grant may be relativelyinsufficient.

Meanwhile, as to the FDD cell, instead of allowing a UL transmission inall UL subframes of a UL carrier, it may also be restricted such thatthe UL transmission is scheduled only in some UL subframes. For example,it may be restricted such that some of UL subframes of the FDD cell arechanged to be used in a DL transmission in order to support more DLtraffics or that some of UL subframes are not used in the ULtransmission in order to regulate an inter-cell interference. In thiscase, an aperiodic SRS request may be included in DL scheduling DCI forthe FDD cell. Whether the aperiodic SRS request is included in the DLscheduling DCI for scheduling the FDD cell may be given by RRCsignaling.

Hereinafter, a collision of an UpPTS and/or a PUCCH in a SRStransmission is described.

When a plurality of serving cells are configured to a wireless device, aPUCCH transmission of a first serving cell and a SRS transmission of asecond serving cell may overlap in the same subframe. In this case, theSRS transmission is discarded in a corresponding subframe, and only aPUCCH is transmitted. This is to decrease a peak-to-average power ratio(PAPR). Alternatively, if a shortened PUCCH is configured, since a SRSsymbol and the PUCCH do not overlap in a corresponding subframe, both ofa PUCCH of the first serving cell and a SRS of the second serving cellmay be transmitted.

When a TDD cell and an FDD cell are configured to the wireless device,it may be configured such that a periodic SRS is transmitted by using anUpPTS of an S subframe of the TDD cell. In this case, a PUCCHtransmission of the FDD cell and a SRS transmission in an UpPTS of theTDD cell may collide in one subframe. If the UpPTS consists of last 2OFDM symbols of the S subframe and if it is configured to transmit a SRSin both of the 2 OFDM symbols, a PUCCH transmission is allowed tooverlap in one of the two OFDM symbols for the UpPTS even if theshortened PUCCH is applied. When such a collision occurs, the followingmethods are proposed to avoid an increase in a PAPR.

In a first embodiment, if it is configured to transmit a SRS by using anUpPTS, a shortened PUCCH is not configured. That is, the SRStransmission may be discarded.

In a second embodiment, when a shortened PUCCH is applied, it is notconfigured to transmit a SRS in an OFDM symbol for an UpPTS. When theshortened PUCCH is applied, it is not allowed to transmit the SRS in theOFDM symbol for the UpPTS.

In a third embodiment, in case of an FDD-TDD cell aggregation, if it isconfigured to transmit a PUCCH to an FDD cell, it is not configured totransmit a SRS by using an UpPTS of a TDD cell. This is a case where theFDD cell is a primary cell.

The above operation may be understood not only from a perspective of anetwork configuration but also from a perspective of not expecting acorresponding configuration by the wireless device.

The above operation may apply not only to a case where an SRS istransmitted in 2 OFDM symbols of an UpPTS or a case where the SRS istransmitted in a first OFDM symbol of the UpPTS but also to all cases ofbeing configured to transmit the SRS by using the UpPTS.

The above operation may apply not only to a periodic SRS but also to anaperiodic SRS.

FIG. 5 is a block diagram showing a wireless communication systemaccording to an embodiment of the present invention.

A BS 50 includes a processor 51, a memory 52, and a radio frequency (RF)unit 53. The memory 52 coupled to the processor 51 is driven by theprocessor 51, and stores various instructions for performing anoperation of the BS 50. The RF unit 53 is coupled to the processor 51,and transmits and/or receives a radio signal. The processor 51implements the proposed functions, procedures, and/or methods. In theaforementioned embodiment, a plurality of serving cells may becontrolled/managed by the BS 50, and an operation of one or more cellsmay be implemented by the processor 51.

A wireless device 60 includes a processor 61, a memory 62, and an RFunit 63. The memory 62 is coupled to the processor 61, and storesvarious instructions for performing an operation of the wireless device60. The RF unit 63 is coupled to the processor 61, and transmits and/orreceives a radio signal. The processor 61 implements the proposedfunctions, procedures, and/or methods. In the aforementioned embodiment,an operation of the wireless device may be implemented by the processor61.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

In the above exemplary systems, although the methods have been describedon the basis of the flowcharts using a series of the steps or blocks,the present invention is not limited to the sequence of the steps, andsome of the steps may be performed at different sequences from theremaining steps or may be performed simultaneously with the remainingsteps. Furthermore, those skilled in the art will understand that thesteps shown in the flowcharts are not exclusive and may include othersteps or one or more steps of the flowcharts may be deleted withoutaffecting the scope of the present invention.

What is claimed is:
 1. A method for receiving a sounding referencesignal (SRS) in a wireless communication system, the method beingperformed by a base station that manages a plurality of serving cellsincluding a time division duplex (TDD) cell and a frequency divisionduplex (FDD) cell, the method comprising: transmitting downlink controlinformation to a user equipment at a first serving cell of the pluralityof serving cells, the downlink control information including a carrierindicator and a downlink assignment, the carrier indicator indicating asecond serving cell of the plurality of serving cells, the downlinkassignment indicating a downlink resource assignment at the secondserving cell; and when the downlink control information includes anaperiodic SRS request that indicates a triggering of an SRStransmission, receiving an SRS from the user equipment at the secondserving cell, wherein: if the first serving cell is the FDD cell and thesecond serving cell is the TDD cell, the downlink control informationincludes the aperiodic SRS request, and if the first serving cell is theTDD cell and the second serving cell is the FDD cell, the downlinkcontrol information does not include the aperiodic SRS request.
 2. Themethod of claim 1, further comprising: configuring the plurality ofserving cells to the user equipment.
 3. The method of claim 1, whereinthe aperiodic SRS request in the downlink control information has onebit.
 4. The method of claim 3, wherein, when the bit is set to one, theaperiodic SRS request indicates the triggering of the SRS transmission.5. The method of claim 1, wherein the first serving cell is a primarycell and the second serving cell is a secondary cell.
 6. The method ofclaim 1, wherein the downlink control information is transmitted on aphysical downlink control channel (PDCCH) of the first serving cell. 7.The method of claim 1, further comprising: transmitting an SRSconfiguration for the SRS transmission to the user equipment, the SRSconfiguration including a bandwidth of the SRS and a subframe offset ofthe SRS.
 8. A device configured to receive a sounding reference signal(SRS) in a wireless communication system and manage a plurality ofserving cells including a time division duplex (TDD) cell and afrequency division duplex (FDD) cell, the device comprising: a radiofrequency (RF) unit configured to receive and transmit radio signals;and a processor operatively coupled with the RF unit and configured to:control the RF unit to transmit downlink control information to a userequipment at a first serving cell of the plurality of serving cells, thedownlink control information including a carrier indicator and adownlink assignment, the carrier indicator indicating a second servingcell of the plurality of serving cells, the downlink assignmentindicating a downlink resource assignment at the second serving cell,and control the RF unit to receive an SRS from the user equipment at thesecond serving cell when the downlink control information includes anaperiodic SRS request that indicates a triggering of an SRStransmission, wherein: if the first serving cell is the FDD cell and thesecond serving cell is the TDD cell, the downlink control informationincludes the aperiodic SRS request, and if the first serving cell is theTDD cell and the second serving cell is the FDD cell, the downlinkcontrol information does not include the aperiodic SRS request.
 9. Thedevice of claim 8, wherein the processor is further configured toconfigure the plurality of serving cells to the user equipment.
 10. Thedevice of claim 8, wherein the aperiodic SRS request in the downlinkcontrol information has one bit.
 11. The device of claim 10, wherein,when the bit is set to one, the aperiodic SRS request indicates thetriggering of the SRS transmission.
 12. The device of claim 8, whereinthe first serving cell is a primary cell and the second serving cell isa secondary cell.
 13. The device of claim 8, wherein the downlinkcontrol information is transmitted on a physical downlink controlchannel (PDCCH) of the first serving cell.